Electrospinning is known as a method for manufacturing filamentous (fibrous form) substances (nanofibers) made of resin or the like and having a diameter in a submicron scale.
In the electrospinning method, nanofibers are manufactured by effusing (ejecting) a solution which is a raw material liquid into a space through a nozzle or the like, while charging the solution by applying an electric charge so as to cause the solution traveling the space to undergo the electrostatic stretching phenomenon. Here, the solution is prepared by dispersing or dissolving resin or the like in a solvent.
More specifically, the volume of the electrically charged and effused solution decreases as the solvent evaporates from the solution traveling the space. On the other hand, the electric charge applied to the solution remains in the solution. As a result, charge density of the particles of the solution traveling the space increases. Since the solvent in the solution continuously evaporates, the charge density of the solution further increases. When Coulomb force, which is generated in the solution and acts oppositely, exceeds the surface tension of the solution, the solution undergoes a phenomenon in which the solution is explosively stretched into filament (electrostatic stretching phenomenon). Such electrostatic stretching phenomenon repeatedly occurs at an exponential rate in the space, thereby manufacturing nanofibers made of resin with a submicron diameter (for example, see Patent Reference 3).
The solvent for the solution used in such a method needs to be easily volatilized. Liquids having such properties are typically organic solvents in light of availability, cost and the like. However, most organic solvents are flammable. Therefore, taking measures to prevent the evaporated solvent from exploding is an important concern.
In view of such concerns, there is a proposed method for preventing explosions by closing the space where the solvent evaporates and filling the space with inert gas such as nitrogen so as to remove, from the space, oxygen that causes explosions (for example, see Patent Reference 1).
Further, a thin film having three dimensional structure of three dimensional mesh can be obtained by depositing nanofibers thus manufactured on a deposition member or the like. Further, by depositing the nanofibers thicker, a highly porous web having submicron mesh can be manufactured. Thus manufactured thin film and highly porous web can be preferably applied to a filter, a separator for use in a battery, a resin electrolyte membrane or an electrode for use in a fuel cell, or the like. Such applications of the highly porous web made of the nanofibers are expected to significantly improve performances of those devices.
Conventionally, when manufacturing such web made of the nanofibers, as disclosed in Patent Reference 2, an elongated highly porous web is manufactured by depositing nanofibers on an elongated band shaped deposition member which is wound around a winding member, and collecting the deposition member along with the nanofibers deposited thereon. When there is no more deposition member to be supplied, it is replaced with a new deposition member, and a highly porous web made of nanofibers is manufactured.
The nanofibers manufactured in the space are deposited and used as a nonwoven fabric in some cases. In this case, uniform thickness of the nonwoven fabric and uniform diameter of the nanofibers making up the nonwoven fabric are required. Thus, the inventors of the present application have previously proposed a nanofiber manufacturing apparatus which can provide spatially even distribution of nanofibers by transporting the nanofibers by gas flow, and diffusing the nanofibers together with the gas flow. By depositing the spatially and evenly distributed nanofibers, a nonwoven fabric having two-dimensionally uniform quality can be manufactured.    Patent Reference 1: Japanese Unexamined Patent Application Publication No. 2-273566    Patent Reference 2: Japanese Unexamined Patent Application Publication No. 2006-37329    Patent Reference 3: Japanese Unexamined Patent Application Publication No. 2004-238749